Conventionally, control techniques for varying a differential pressure between a load pressure of work machine actuators and a discharge pressure of a hydraulic pump according to an externally designated work mode indicating a type of work in order to obtain an operability for operation levers that corresponds to the nature of the work of a construction machine have been disclosed in, for example, Japanese Patent Application Laid-Open No. 2-76904.
With this publicly disclosed technology, it is possible, when the work mode is varied from "normal work" mode to "micro operation" mode, to perform finer work suitable for the "micro operation" mode by making the aforementioned differential pressure smaller than in the case of the "normal work," and by making the change in the drive velocity of the work machine actuators per fixed amount of operation of the operation levers smaller than in the case of the "normal work."
Japanese Patent Application Laid-Open No. 2-164941 also discloses this type of control system, and involves effecting control such that the aforementioned differential pressure is reduced in accordance with a reduction in the rotational speed of the engine, thereby increasing the so-called metering region, which decreases as the engine rotational speed decreases (or, to put it another way, decreases the dead band that increases as the rotational speed decreases), and thus improving the operability of the operation levers.
In this way, these conventional techniques are control methods that vary the differential pressure depending on the work mode or engine rotational speed, that thereby vary the relationship (hereafter referred to as the "operating characteristics") between the work machine actuator velocity and the operation amounts of the operation levers, and that consequently improve the operability of the operation levers; these conventional techniques, however, only involve a one-to-one correspondence between the change in the differential pressure and the work mode or engine rotational speed, and do not involve controlling the correlation with the actual load on the work machine actuators.
However, hydraulic shovels and the like are generally equipped with a device that effects equivalent horsepower control or the like so that the absorbing torque of a hydraulic pump is matched with the engine output torque (see FIG. 7(c)), and that controls the discharge quantity of the hydraulic pump according to the so-called PQ curve of FIG. 7(b) so that the absorbing torque value at this matching point is obtained. Such devices are well-known devices which are constructed with, for example, a TVC valve as a main component.
However, when control is effected in this way so that the torque is kept at or below a certain value, then when the hydraulic pump discharge pressure Pp, that is, the load on the work machine actuators, is high, the hydraulic pump quantity of discharge Qp is low, as shown by the P2 (&gt;P1) of FIG. 7(b). For this reason, as shown in FIG. 7(a), compared to when the load pressure Pp is, on the contrary, low, at P1, when the load pressure Pp is high, at P2, the work machine drive velocity v is affected by the low quantity of discharge Qp, and is kept low, as shown by the break line, and the dead band increases and lever operation is greatly degraded.
With the conventional techniques, moreover, controlling the differential pressure according to the work mode does not involve effecting control according to the actual work machine drive state pertaining to the work machine. Specifically, when there are a plurality of work machines, the operating characteristics required for each one is different, and it is not possible to meet these requirements by setting a one-to-one correspondence between the differential pressure and the work mode. An example of a case in which there are different requirements is one in which work machine actuators for excavation should be driven as "normal work," but since the shape of the land is irregular, work machine actuators for travel should be driven as "micro operation." In this case, in the past, the work mode designation had to be varied manually every time there was a switch from excavation work to travel work, and vice versa, which was inconvenient in that it complicated operation and increased the burden on the operator.
With the foregoing in view, the first object of the present invention is to provide a controller that affords better operability than in the past by controlling the aforementioned differential pressure according to the drive states and the like of the individual work machines or according to the load on the work machine actuators.
The aforementioned prior art, moreover, only involves varying the differential pressure in a one-to-one correspondence with the work mode or engine rotational speed, and does not involve effecting control by taking into account the effects of pressure oil leaks in the actual hydraulic circuit.
Specifically, an increase in the load on the work machine actuators is accompanied by an increase in pressure oil leakage in the hydraulic pipe lines between the work machine actuators and the operation valves (flow rate control valves), and thus by a substantial decrease in the volume efficiency of the hydraulic pump. A reduction in engine rotational speed, moreover, is accompanied by an increase in the ratio of the leakage flow rate to the pump discharge flow rate, and thus by a marked decrease in the aforementioned volume efficiency. The actual velocity of the work machine actuators is therefore decreased, and the relationship with the actual operating characteristics is considerably varied. The desired operating characteristics consequently cannot be obtained, and the operability is degraded.
With the conventional techniques, moreover, decreasing the differential pressure according to the work mode or the engine rotational speed does not involve effecting control according to the actual operating conditions of the operation levers. For example, when all of a plurality of operation valves (flow rate control valves) are in a neutral position and a conventional technique is used directly, a phenomenon called "jumping," in which the work machine actuators move suddenly when operation lever operation is started, occurs at high engine rotational speeds, and at low engine rotational speeds an increase in dead time or dead band occurs when operation lever operation is started; in either case, the operability is degraded.
With the foregoing in view, the second object of the present invention is to provide a device that does not undergo any operability degradation even in the event of a pressure oil leak, and that does not undergo any operability degradation when the operation levers are operated from a neutral position.
The aforementioned conventional techniques involve nothing more than varying the differential pressure in a one-to-one correspondence with the work mode or engine rotational speed, and are not based on the premise of matching the engine output torque with the hydraulic pump absorbing torque.
Accordingly, when applied to a hydraulic shovel or the like, which has engine output torque limitations, engine failure or the like occurs when the load on the work machines becomes great, and work therefore cannot be continued, which is inconvenient.
With the foregoing in view, the third object of the present invention is to provide a device that makes it possible to prevent inconveniences such as engine failure and that improves operability by controlling the aforementioned differential pressure while matching the engine output torque and the hydraulic pump absorbing torque.
The aforementioned conventional techniques, moreover, specifically involve providing a valve for differential pressure control to the hydraulic circuit that controls the hydraulic pump, and controlling the hydraulic pump swash plate swash angle by means of this differential pressure control valve so that a differential pressure corresponding to the engine rotational speed or work mode may be obtained. Such swash plate control, however, is not based on the premise that the engine output torque matches the hydraulic pump absorbing torque.
Consequently, when applied as is to a hydraulic shovel or the like that has engine output torque limitations, engine failure or the like occurs when the load on the work machine becomes considerable, and work cannot be continued, which is inconvenient.
On the other hand, with a hydraulic shovel or the like, there are cases in which a control valve for controlling the pump absorbing torque that controls the hydraulic pump swash plate swash angle so that the hydraulic pump absorbing torque matches the engine output torque is provided to the hydraulic circuit that controls the hydraulic pump. However, when the absorbing torque control valve and the differential pressure control valve are both present, but with no interrelationship, and the hydraulic pump is controlled, there are cases in which, depending on the operating conditions, the torque limitations come into play, and the operation lever operability is degraded.
With the foregoing in view, the fourth object of the present invention is to provide a device that makes it possible to prevent such inconveniences as engine failure, and to thereby improve operability, by controlling the differential pressure with the differential pressure control valve, taking into account the aforementioned absorbing torque, while matching the engine output torque with the hydraulic pump absorbing torque by means of the absorbing torque control valve.